I'm having a problem similar to one you had years ago...

I'm developing a tool to store the biometric data of a signer inside the PDF along with the digital signature, but I'm not finding where to store them.

At first I saved them in custom metadata, but I don't think that's good practice…

I'm trying to store them in the signature dictionary, but Adobe tells me the signature is invalid.
I also tried saving them in an extended DSS, but if someone changes it, the signature wouldn't be invalidated.

I understand that the correct approach would be to store them somewhere in the self-contained PDF, encrypted, so that if it's modified the signature becomes invalid…

After so many years, were you able to find a correct way to do it? Thank you very much!